Low-level wind shear (LLWS), a phenomenon associated, for example, with weather fronts, thunderstorms, and low-level jets, poses a major safety hazard for commercial and general aviation. In scheduled air carriers, LLWS is responsible for the most fatalities in the vicinity of air terminals, where aircraft are most vulnerable during landing and takeoff. Furthermore, the potential for tragedy increases each year as the density of air traffic increases.
As a basis for enabling determination of the existence of LLWS as a hazard threat to an aircraft, a function known commonly as a "hazard index" has been developed which produces a warning factor F which is expressible as the following function of the horizontal wind acceleration component Wx, the vertical wind velocity component V airspeed As, and gravitational acceleration g: EQU F=Wx/g+V/As. [1]
The maximum permissible value of warning factor F is determined in relation to the particular performance capabilities of an aircraft and this maximum permissible value F can be used as an alert and warning threshold. For example, inertial reactive systems have been developed (such as that manufactured by Safe Flight Instrument Corporation of White Plains, N.Y.) which utilize the inertial components of an aircraft responding to the wind components Wx and V to produce the warning factor F, and to announce when the aircraft has already encountered LLWS. However, while an inertial reactive system using aircraft acceleration to detect LLWS is helpful, its response time is inherently limited, as is its sensitivity, due to the fact that the large mass of the aircraft is, itself, the sensor. Thus, there is a need for an LLWS warning system which is more sensitive than an inertial reactive system and for one which will be able to remotely predict an LLWS encounter rather than merely react to the encountering of LLWS.
However, the remote sensing of wind velocity, for use in application of the above-noted hazard index, does not represent, at present, a suitable technique for aircraftborne applications. That is, in general, long-range measurement of wind velocity requires the use of radar, while short-range measurements require the use of lidar or sodar. However, radar poses the disadvantages that suitable systems are comparatively large and expensive; antenna side lobes limit usefulness close to the ground; and clear-air targets require high transmitter power. Sodar is prone to problems related to the fact that it sometimes only senses wind in special layers and it is sensitive to noise from precipitation, high wind, and vehicles; while lidar poses a possible danger to eyes and its beam is attenuated by clouds and fog. On the other hand, since many airports will never have enough sophisticated equipment to remotely detect areas of LLWS, the need for aircraftborne LLWS detection systems cannot be eliminated by the development of land based ones.
Another approach that has been taken to the problem of detecting LLWS with sufficient warning, instead of utilizing the above-noted hazard index based on wind speed components, utilizes detection of the existence of temperature gradients, as exist in LLWS turbulence, as a hazard indicating factor. For example, in the present applicant's earlier U.S. Pat. No. 4,342,912, an apparatus for detecting air disturbances created by low level wind shear is disclosed that utilizes an infrared scanning radiometer which uses infrared filters having different distance sensitivities and through which infrared radiation is directed to a radiation sensor which develops signals corresponding to air temperatures at a distance from the aircraft. The temperatures are used with an equation to give an advance warning of an air turbulence hazard to the aircraft, such as due to LLWS, when the temperature gradients detected so indicate. For example, a warning may be issued if the temperatures sensed represent a sustained temperature drop for a predetermined time of at least a predetermined rate.
The use of temperature detection as an indicator of LLWS not only presents an advantage in that airborne, remote temperature sensing techniques may be implemented in a practical manner, but also benefits from the fact that a temperature decrease precedes the winds of an LLWS, offering increased response time. Even though such infrared detecting apparatus can effectively measure temperature gradients associated with LLWS, generating an indication of LLWS on the basis of only temperature related values will not provide the degree of sensitivity and reliability (in terms of minimizing both the number of instances of LLWS that are missed and the number of instances where the existence of LLWS is falsely indicated) as would be achievable using the above-noted, established function for determining the hazard factor F of equation [1] above.
Apart from the available devices for detecting LLWS, various empirical and model atmospheric studies have been conducted which yield functional relationships between wind speed and temperature. In accordance with a model developed by NASA, the following relation exists between maximum temperature drop and peak outflow speed, U.sub.max ; where .DELTA.T is the temperature drop measured between the temperature of an air parcel within a low-level wind shear event and the ambient temperature enveloping the event: EQU U.sub.max =2.5 .DELTA.T [2] EQU (MKS units)
Additionally, Donald S. Foster, in an article published in the Monthly Weather Review, Volume 86, No. 3. Mar. 1958, entitled "Thunderstorm Gusts Compared with Computed Downdraft Speeds," reported on a study performed for purposes of forecasting maximum wind gusts expected to accompany thunderstorm activity. In this report, a theoretical downdraft speed relationship was proposed with a vertical velocity W.sub.o being approximated by the relationship between (a) the above-mentioned temperature drop .DELTA.T, (b) the distance Z the above-mentioned air parcel drops, this distance being further defined in the Foster article, (c) the ambient temperature T.sub.m and (d) the acceleration of gravity of: ##EQU1## With respect to the results reported by Foster, a correlation was found, of the speed of the computed downdrafts relative to the gust velocity accompanying the thunderstorms measured, that was statistically significant, even though the correlation (0.50) was not very high, i.e., a significant variation between predicated and actual values existed, resulting in an average computed downdraft velocity of 78 knots relative to an average measured gust velocity of only 61 knots.